When I first received this amp from aliexpress, one of the first things I noticed after I took it out of the packaging was a slight rattle when shook. I decided to investigate, taking apart the amp, and the only thing I could make out that was rattling was either the battery or the wires connecting to the battery (see below pic). Really didn't do much adjusting at all, just a little nudge to the wires and the battery, and, after putting it back together, the rattle was gone!

Sound Quality

better than expected, was rather skeptical before purchasing

nice, smooth lows, doesn't seem to modify their sound very much at all

the lower mids are a little more pronounced, while the upper mids and highs seem to be negligibly affected

great for the price!

On aliexpress and elsewhere, the U3 Mini is advertised as having a warm sound, but from what I have experienced so far, it really seems to be more of a warm-ISH sound. Yes, it is not bright, but I would not say that it is very warm either.

Conclusion

Would recommend this product if you are looking for a portable amp in this price range! Design and sound are very nice, and for only ~$35, you can get a decent amp to power your cans!

HEY just got bluebird U3.6 and i am amazed and astonished by the sound quality and the soundstage with the original op amp
Anyway i also ordered with that muses02 chip to upgrade original amp but till now seems very hard to unscrew one of the four screws in the upper surface of bluebird close to the volume knob....
Tried any kind of screwdrivers but that screw cant be rotated no matter how hard i have tried...
Any ideas?